Current phosphate-bonded investments and refractory die materials utilize fillers selected from quartz, tridymite and cristobalite to provide a degree of thermal expansion which is acceptable for sintering conventional porcelains having percent thermal expansion values lower than 0.65% in the temperature range of 20.degree.-500.degree. C. These fillers are combined with binders containing magnesium oxide, mono- or di-ammonium phosphate and colloidal-silica suspensions which provide physical expansion and strength. Trace amounts of surfactants are also added to control dispersion and surface properties of the fillers and binders. Problems exist, however, when such filler and binder combinations are used in developing phosphate-bonded investments and refractory die materials for higher expansion dental porcelains.
The primary difference between investments and refractory die materials lies in the selection of the particle size distribution of the fillers. Investments usually have a coarser particle size distribution. Investments are primarily used as the mold materials for the casting of dental alloys using lost wax processes. Refractory die materials are conventionally utilized as substrates for sintering dental porcelains and alloy powders and thus are subjected to high temperature firing cycles. Both investments and refractory die materials are formulated to provide proper setting, thermal and net positive physical expansion.
Although the silica-based fillers are inexpensive, they suffer from phase transformations at their transformation temperatures. These phase transformations cause sudden and large changes in the thermal expansion of the fillers. FIG. 1 is a graph showing thermal expansion verse temperature profiles for some of the industrial filler materials. It is apparent that the three forms of silica--quartz, tridymite and cristobalite--undergo phase transformation from an alpha form to a beta form. The profiles indicate that each transformation is accompanied by a large change in thermal expansion at the transformation temperature. For example, alpha-quartz converts to beta-quartz at about 573.degree. C. with a thermal expansion change of about 0.5%. Similarly, alpha-cristobalite transforms to a beta-form at a temperature between about 200.degree. C. and about 270.degree. C., and produces a thermal expansion change of about 1.0%. The beta forms are stable only above the transformation temperatures. Upon cooling each composition, an inversion back to the lower alpha-form occurs.
Usually, a proper proportioning of these allotropes is controlled to arrive at the desired thermal expansion verse temperature profile to match the intended application. The addition of binders has only a minor influence on the thermal expansion behavior.
Low temperature phase transformation associated with the use of alpha-cristobalite, in most instances, does not pose problems in investments used for obtaining dense casting of dental alloys using the lost-wax technique. However, phosphate-bonded refractory die materials containing these different allotropes of silica as fillers exhibit the sudden and large changes (referred to herein as spikes) at the phase transformation temperatures. When used as a die (substrate) for sintering dental porcelain and alloy powders, compositions containing such fillers are prone to the formation of micro and macro-defects in the overlaid green (unsintered) products. These defects, if not repaired during sintering, get carried into the final sintered products.
The above phenomena is especially prevalent where the overlaid layers differ considerably in their thermal expansion values when compared to the thermal expansion of the die-material. The use of alpha-quartz alone as a filler appears to be acceptable for dental porcelains having low thermal expansions, e.g.&lt;0.65% in the 25.degree.-500.degree. C. range. However, many of today's dental porcelains exhibit thermal expansions of greater than 0.80%. In fact, many of the dental porcelains currently being used have thermal expansions of between 0.84 and 0.87%. One such class of porcelains is Optec.RTM. porcelains, available from Jeneric/Pentron, Wallingford, Conn. Due to the strength of some of these porcelains, crowns and bridges have been produced without the need for metal substrates.
In order to accommodate the sintering of higher expanding dental porcelain and alloy powders, efforts have been made to replace part of the alpha-quartz filler with an alpha-cristobalite filler. These attempts have been unsuccessful though since the presence of cristobalite produces a spike in the thermal-expansion verse temperature profile in the range of 200.degree. to 270.degree. C. The spike produces interfacial tensile stresses between the substrate and the green overlay which is inevitably detrimental to the integrity of the final restoration. Theoretically, it is preferred to have a compressive mode of stress at the interface of the die material and the overlaid material to eliminate defects in the final restoration. This is accomplished by having a refractory die material which has higher thermal expansion values than those of the materials being sintered thereon.